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Halo Coronal Mass Ejections and Geomagnetic Storms LETTER Earth Planets Space, 61, 1–3, 2009 Halo coronal mass ejections and geomagnetic storms Nat Gopalswamy NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A. (Received June 21, 2008; Revised xxxx xx, 2008; Accepted October 27, 2008; Online published xxxx xx, 2009) In this letter, I show that the discrepancies in the geoeffectiveness of halo coronal mass ejections (CMEs) re- ported in the literature arise due to the varied definitions of halo CMEs used by different authors. In particular, I show that the low geoeffectiveness rate is a direct consequence of including partial halo CMEs. The geoeffec- tiveness of partial halo CMEs is lower because they are of low speed and likely to make a glancing impact on Earth. Key words: Coronal mass ejections, geomagnetic storms, geoeffectiveness, halo CMEs. 1. Introduction erage speed is ∼1000 km/s compared to ∼470 km/s for or- Coronal mass ejections (CMEs) that appear to surround dinary CMEs), so the V part is expected to be high. When the occulting disk of the observing coronagraphs in sky- CMEs are aimed directly at Earth, the ICMEs are likely to plane projection are known as halo CMEs (Howard et al., arrive at Earth as magnetic clouds (MCs), which are a subset 1982). Halo CMEs are fast and wide on the average and are of ICMEs that have flux rope structure. Since Earth passes associated with flares of greater X-ray importance because through the nose of the MC, the chance of encountering Bs only energetic CMEs expand rapidly to appear above the somewhere within the MC is high (except for unipolar MCs occulting disk early in the event (Gopalswamy et al., 2007). with north-pointing axis—see Yurchyshyn et al., 2001). Extensive observations from the Solar and Heliospheric Ob- When the ICME is shock-driving, the sheath portion lying servatory (SOHO) mission’s Large Angle and Spectromet- between the shock and the MC may also play a significant ric Coronagraphs (LASCO) have shown that full halos con- role in producing geomagnetic storms (see e.g., Gosling stitute ∼3.6% of all CMEs, while CMEs with width ≥120◦ et al., 1990). The sheath may have intervals of north and account for ∼11% (Gopalswamy, 2004). Full halos have south-pointing magnetic fields, in addition to the varying an apparent width (W)of360◦, while partial halos have field orientation in the MC portion. An Earth-directed halo 120◦ ≤ W < 360◦. Halo CMEs are said to be frontsided if CME leads to a situation whereby Earth passes through the the site of eruption (also known as the solar source) can be nose of the shock, where the sheath field is most intense and identified on the visible disk usually identified as the loca- may cause intense magnetic storm if south-pointing. Thus tion of H-alpha flares or filament eruptions. Details on how the ability of a halo CME in producing a geomagnetic storm to identify the solar sources can be found in Gopalswamy depends on the structure of its interplanetary counterpart et al. (2007). Halos with their sources within ±45◦ of the (the ICME event). central meridian are known as disk halos, while those with a Since halos became common place in the SOHO era, central meridian distance (CMD) beyond ±45◦ but not be- there have been several attempts to characterize their geo- yond ±90◦ are known as limb halos. Disk halos are likely effectiveness (see e.g., Zhao and Webb, 2003; Yermolaev to arrive at Earth and cause geomagnetic storms, while limb and Yermolaev, 2003; Kim et al., 2005; Yermolaev et al., halos only impact Earth with their flanks and hence are less 2005; Gopalswamy et al., 2007). Using CMEs from the rise geoeffective (see Gopalswamy et al., 2007). phase of solar cycle 23, St. Cyr et al. (2000) concluded that Since CMEs propagate approximately radially from the ∼75% of the frontside CMEs are geoeffective. In most of Sun (except for a small eastward deflection due to solar the works discussed here, the geoeffectiveness is defined as rotation—see Gosling et al., 1987), disk halos are likely hit the ability of a CME or an interplanetary structure to cause Earth. Of course, the interplanetary counterpart of CMEs geomagnetic storms with intensity (Dst) ≤−50 nT. While (ICMEs) must contain southward magnetic field component most of the intense storms are caused by CMEs, moderate (Bs ) to be geoeffective. It is well known that the intensity storms (−100 nT < Dst < −50 nT) may also be caused of the resulting magnetic storm depends on the magnitude by other structures such as corotating interaction regions. of Bs and the speed V with which the CME impacts Earth’s Zhao and Webb (2003) found an overall geoeffectiveness magnetosphere (see e.g., Gonzalez et al., 1994; Tsurutani rate of ∼64% for frontside halos detected up to the solar and Gonzalez, 1997). Halo CMEs are more energetic (av- maximum in 2000. Yermolaev and Yermolaev (2003) used data from the period 1976–2000 and came up with a lower Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sci- ∼ ences (SGEPSS); The Seismological Society of Japan; The Volcanological Society rate of 40–50%. Michalek et al. (2006) found that 56% of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sci- of frontside halos are geoeffective, but they did not use all ences; TERRAPUB. No claim is made to original U.S. Government works. the halos because of limitations in the method of obtaining 1 2 N. GOPALSWAMY: HALO CORONAL MASS EJECTIONS space speeds. Kim et al. (2005) reported that only about all CMEs are full halos, while 11% are full + partial halos 40% of the frontside halos are geoeffective. Yermolaev et (Gopalswamy, 2004). Assuming that these fractions apply al. (2005) compiled published results and noted that that equally well to the front and backside CMEs and recall- the geoeffectiveness rate varied from <40% to >80%. Re- ing that 229 full halos were frontsided, one can estimate cently Gopalswamy et al. (2007) analyzed 378 halo CMEs the frontside wide CMEs as (11%/3.5%) × 229 = 720. covering almost whole of solar cycle 23 and found that Therefore, 720 − 229 = 491 is the likely number of partial ∼71% of frontside halos are geoeffective. Zhao and Webb halos. At the 27% geoeffectiveness rate estimated above, (2003) and Gopalswamy et al. (2007) used the same defini- 134 of the 491 partial halos are expected to be geoeffec- tion of halo CMEs (W = 360◦) and obtained similar geo- tive. Therefore, out of the 720 wide CMEs, 163 full halos effectiveness rates. On the other hand, Kim et al. (2005) and 134 partial halos (total of 297 or 41%) are geoeffec- and Yermolaev and Yermolaev (2003) defined their halos tive. This is virtually the same as the 40% rate obtained by as CMEs with W ≥ 120◦ and obtained the lower geoef- Kim et al. (2005) and confirms that inclusion of partial ha- fectiveness rate. The purpose of this letter is to show that los lowers the overall geoeffectiveness rate. Note that the the discrepancy in the rate of geoeffectiveness can simply geoeffectiveness of halos varies with the phase of the so- be explained by the different definition of halo CMEs used lar cycle (see Gopalswamy et al., 2007, figure 8; Zhao and by these authors. We take Kim et al. (2005) and Gopal- Webb, 2003) and the way multiple halos associated with a swamy et al. (2007) representing the low and high geoef- given storm are treated. We estimate that these effects can fectiveness rates, respectively. Both these works have pub- account for an additional 10% variation. lished their list of events and the study periods have maxi- 2.1 Why are partial halos less geoeffective? mum overlap. They also use data from the same instrument In studying the geoeffectiveness of CMEs as a function (SOHO/LASCO), so the comparison is straightforward. Fi- of source latitude, Gopalswamy et al. (2007) found that nally, both works use the same definition of geoeffective- the strongly geoeffective (Dst ≤−100 nT) and moder- ness: the ability of a CME to produce a geomagnetic storm ately geoeffective (−50 ≥ Dst > −100 nT) CMEs have with Dst ≤−50 nT. average longitudes of W10 and E03, respectively. The non-geoeffective CMEs have an average longitude similar 2. Geoeffectiveness of Partial Halo CMEs to the moderately geoeffective CMEs (E02). Furthermore, Kim et al. (2005) investigated 305 CMEs (1997 to 2003) the fraction of limb halos steadily increases from 17% for that included full (W = 360◦) and partial halos (120◦ < strongly-geoeffective, to 31% for moderately geoeffective, W < 360◦) and found that 121 of them were geoeffective. and 37% for non-geoeffective CMEs. The average speeds On the other hand Gopalswamy et al. (2007) studied 378 also decrease in the same order (see figure 9 of Gopalswamy full halos for the period 1996 to 2005. For making a proper et al., 2007). From these observations, Gopalswamy et comparison, we first separate the full and partial halos in al. (2007) concluded that non-geoeffective CMEs are rel- Kim et al. (2005). We then use the geoeffectiveness of a atively slower, originate predominantly from the eastern subset of full halos from Gopalswamy et al. (2007) corre- hemisphere, and have a greater central meridian distance. sponding to the Kim et al. (2005) study period to estimate Partial halos generally have properties similar to the mod- the geoeffectiveness of partial halo CMEs. erately geoeffective and non-geoeffective halos. Thus par- Among the 378 full halos reported by Gopalswamy et tial halos are less energetic and do not expand enough to al.
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